1. Field of the Invention
This invention relates to an electrographic process and apparatus by which it is carried out.
2. Description of the Prior Art
In general, a process of converting a negative picture image into a positive picture image is realized by a reversal development process or a special electrographic process which makes use of a photosensitive plate composed of a photosensitive layer and an insulating layer coated thereon and in which a corona charge polarity at each charging step of producing on the insulating layer an electrostatic latent image is suitably selected. Another well known process is realized by using an electrographic photosensitive screen and by suitably selecting at each electrographic step a corona charge polarity and a bias voltage.
However, a reversal developing process of converting a latent image which is positive with respect to an optical image into a negative picture image has to adhere a toner to that part of a record medium which is not subjected to an electrostatic attractive force. As a result, there is a risk of an edge effect being induced, so that it is difficult to obtain a good picture image. Particularly, a sufficiently satisfactory reversal developing technique which makes use of one ingredient magnetic toner has not yet been developed and is not yet ready for use in practice.
The above mentioned electrographic process which makes use of the photosensitive plate and in which the corona charge polarity is suitably selected or the above mentioned electrographic process which makes use of the photosensitive screen and in which the bias voltage is suitably selected can obtain a good picture image, but the apparatus by which it is carried out is expensive, large in size and complex in construction. As a result, such electrographic process is not suitable for use in a microfilm printer or the like.